Forced air furnaces for zone controlled systems generally utilize external controls to determine when the furnace will be on and off. In this case, the furnace is generally in either a standby condition, or in an on condition in which it is running at full capacity. No direct control of the heat output rate of the furnace is made at the furnace. This causes duct noise and erratic temperature changes within the zones. The object of Applicants' invention is to control the pressure in the heat exchanger for a more constant output over the normal range of the heating loads. This is accomplished by allowing the furnace to run well below the full firing rate and have the circulation blower running at a reduced speed. The result will be less duct noise, a more constant temperature in the living space, and at low loads the greater on time per cycle will improve air circulation. For example, an electrically-commutated motor (ECM) keeps high efficiency at low speeds, and since the power required varies as the square of the speed, the energy efficiency improves at reduced speeds. It is also expected that the life of the motor and heat exchanger will improve.